What does "not determined" mean?

The raw data provided by 23andMe has undergone a general quality review however only a subset of markers have been individually validated for accuracy. The data from 23andMe’s Browse Raw Data feature is suitable only for informational use and not for medical, diagnostic or other use. Consult with a healthcare professional before making any major lifestyle changes.

In order to ensure that the data we return to customers is highly accurate, we use a stringent algorithm to make genotype calls. Occasionally, a user's data may not allow us to confidently determine his or her genotype at a particular marker. When the algorithm cannot make a confident genotype call, it gives a "not determined" result instead. It is possible that future review will allow us to call the genotype, but until that time, the data does not appear. In downloaded data, the entry for any uncalled SNPs displays '--' instead of a two-letter genotype.